WT1 is a transcription factor originally discovered because of its association with genetically acquired childhood abnormalities, including Wilms' tumor. It has since been discovered that WT1 is a tumor-suppressor gene that if mutated at both alleles in humans increases the probability of acquiring many types of cancer. In addition, WT1 is critical for the proper development of the male genital tract, as humans with WT1 mutations suffer from a variety of male gonadal abnormalities. In mice, WT1 is required for the earliest stage of development of the undifferentiated gonad before sexual differentiation occurs, and then has a later embryonic role directing formation of the male gonad. Intriguingly, WT1 is also expressed at high levels in Sertoli cells in the testis after birth but its function there has not been possible to discern, as WT1-null mice die during embryogenesis. In this proposal, the function of WT1 in postnatal and adult Sertoli cells will be assessed using a new tool. This tool is the Pem homeobox gene proximal promoter (Pem Pp), which is expressed selectively at high levels in postnatal and adult Sertoli cells. This specificity is valuable, as it means that this promoter can be used to selectively express Cre in Sertoli cells and thereby knockout the WT1 gene selectively in these cells and preserve WT1 function elsewhere so that mice can survive to reproductive age. By examining the phenotypic consequences of ablating the WT1 gene in postnatal Sertoli cells, the role of WT1 in spermatogenesis can be determined. Because Pem Pp 5'-flanking sequences of different length (0.3 and 0.6 kb) provide activation of expression at different ages postnatally, this will allow knockout of WT1 in Sertoli cells at different times after birth. Further versatility of expression will be engendered by inclusion of lac repressor (lacR) sequences in the Pem Pp, allowing this promoter to be turned-on at different developmental times after birth in response to the lacR derepressor IPTG. In addition to knocking out WT1 by gene ablation, its function will also be inhibited by dominant-negative WT1 molecules expressed from the Pem Pp in vivo. Although this project is devoted only to studying the function of the WT1 gene, the transgenic mice made to achieve this goal (e.g., those expressing Cre from the Pem Pp) will be a valuable resource for selectively knocking out other genes in postnatal Sertoli cells. [unreadable] [unreadable]